The black comedy of Zoë Heller's Notes on a Scandal delighted critics. A naive school teacher, touched by the plight of a special needs pupil, tries to help him, becomes flattered by his devotion to her, and embarks on a catastrophic affair. Heller is relentless about the consequences but, through the narrator Barbara, also impatient of the sanctimonious and sentimental coverage in the press.

"The sort of young person who becomes involved in this kind of imbroglio is usually pretty wily about sexual matters," Barbara sniffs.

Wily, but as it turns out, fatally immature. Heller was fascinated by the morally grey area of male teenage sexuality. She told me that she had been inspired partly by a buoyant article in the Evening Standard by a male journalist who had been seduced as a teenager. It was important to her that her young male protagonist Connolly was 15, although the producers of the film were anxious about introducing a criminal element.

The finely balanced characterisation and morality of Notes on a Scandal became simplified into a female monster of our time: the cougar.

Predatory women in a position of power or authority; bosses, wealthy people, teachers, mothers, are all on the prowl for young male flesh. Sheba, the teacher in Notes on a Scandal watches Connolly lift his T-shirt to scratch his stomach. She sees how "his pelvic bone jutted out, creating a wide, shallow cavity just above the groin". It is the idealised male form celebrated by today's rich and discerning women. Cougars don't want men, they want boys. The trophy partner should be easily mistaken for your son.

This is the context for the court case involving a 16-year-old special needs pupil from London and his 39-year-old teacher, Teresa McKenzie. The pupil's claim that Mrs McKenzie had seduced him in the lavatories of the British Library caught the imagination of newspapers. Features editors wondered if it would be tasteless to check out the loos, or at least include them in a list of top bizarre places to have sex. The combination of the earnestly educational and the lustful was pure Notes on a Scandal.

Apart from the British Library detail, Mrs McKenzie blurred into the general cast of half-comic, half-pathetic older women infatuated by young men. It was only when the verdict came through last week that we were jolted into a different reality.

Teresa McKenzie was cleared within an hour. The pupil's allegations of a 14-month affair consummated in cars and hotels as well as museum toilets was pure fantasy. Palpably honest and decent, with a fresh, round face and a crucifix round her neck, the 39-year-old mother of two explained that she had tried to help a "deeply disturbed and suicidal teenager".

This was not a grubby game, but reckless altruism of the highest order. "I knew that I was exposing myself to the risk of false accusation," she said, in a low, sensible voice. "I persevered because I was able to distract him from taking his own life."

When we criticise public service as a place of hypocrisy and bloated pensions, remember women such as Mrs McKenzie. My daughter has just returned from a Duke of Edinburgh camp. Teachers cheerfully volunteered to accompany a hormonal and sceptical bunch of teenagers for a week. Imagine the additional risk of supervising the genuinely disturbed, who know their fantasies and paranoia have a willing audience in the legal system. God help our teachers. Nobody else will.

Sarah Sands is deputy editor of the 'London Evening Standard'